Electrochemical studies of single-wall carbon nanotubes as nanometer-sized activators in enzyme-catalyzed reaction

Non-Covalent Functionalization of Carbon Nanotubes for Electrochemical Biosensor Development

Carbon nanotubes (CNTs) have been widely studied and used for the construction of electrochemical biosensors owing to their small size, cylindrical shape, large surface-to-volume ratio, high conductivity and good biocompatibility. In electrochemical biosensors, CNTs serve a dual purpose: they act as immobilization support for biomolecules as well as provide the necessary electrical conductivity for electrochemical transduction. The ability of a recognition molecule to detect the analyte is highly dependent on the type of immobilization used for the attachment of the biomolecule to the CNT surface, a process also known as biofunctionalization. A variety of biofunctionalization methods have been studied and reported including physical adsorption, covalent cross-linking, polymer encapsulation etc. Each method carries its own advantages and limitations. In this review we provide a comprehensive review of non-covalent functionalization of carbon nanotubes with a variety of biomolecules for the development of electrochemical biosensors. This method of immobilization is increasingly being used in bioelectrode development using enzymes for biosensor and biofuel cell applications.

Effect of surfactant type and redox polymer type on single-walled carbon nanotube modified electrodes

Electrodes modified with single-walled carbon nanotubes (SWNTs) offer a number of attractive properties for developing novel electrochemical sensors. A common method to immobilize SWNTs onto the electrode surface is by placing a droplet of a SWNT suspension onto the electrode surface and allowing the solvent to evaporate. In order to maximize the properties of individual SWNTs, surfactants are normally present in these suspensions to provide stable and homogeneous SWNT dispersions. In this study we investigated the effect of different surfactants on the electrochemical and enzymatic performance of SWNT modified glassy carbon electrodes (GCEs). Amperometic biosensors for glucose were fabricated by a two-step procedure. In the first step, SWNT films were deposited onto GCEs by solution casting suspensions of SWNTs in water, Triton X-100, Tween 20, sodium cholate or sodium dodecylbenzenesulfonate (NaDDBS). In the second step, hydrogels containing a redox polymer and the enzyme, glucose oxidase (GOX), were deposited and cross-linked onto the SWNT-modified GCE. Three different redox polymers were tested: 3-ferrocenylpropyl-modified LPEI, (Fc-C3-LPEI), 6-ferrocenylhexyl-modified LPEI, (Fc-C6-LPEI), and poly[(vinylpyridine)Os(bipyridyl)2Cl](2+/3+)(PVP-Os). Biosensors constructed with SWNT films from suspensions of Triton X-100 or Tween 20 generally produced the highest electrochemical and enzymatic responses, with Triton X-100 films producing current densities of ~1.7-2.1 mA/cm(2) for the three different redox polymers. In contrast, biosensors constructed with SWNT films from sodium cholate suspensions resulted in significant decreases in the electrochemical and enzymatic response and in some cases showed no enzymatic activity. The results with SWNT films from NaDDBS suspensions were dependent upon the specific redox polymer used, but in general gave reduced enzymatic responses (~0.05-0.4 mA/cm(2)). These results demonstrate the importance of surfactant type in fabricating SWNT-modified electrode films.

Nitrogen-doped, boron-doped and undoped multiwalled carbon nanotube/polymer composites in WORM memory devices

We report the preparation of write-once-read-many times memory devices using composites of carbon nanotubes and poly(vinyl phenol) sandwiched between Al electrodes. Three types of nanotubes (undoped multiwalled carbon nanotubes, nitrogen-doped multiwalled carbon nanotubes and boron-doped multiwalled carbon nanotubes) are investigated for this application. The OFF to ON state switching threshold is only slightly dependent on nanotube type, but the ON/OFF current ratio depends on both nanotube type and concentration and varies up to 10(6), decreasing for nanotube concentrations larger than 0.50 wt% in the composite.

Effects of Al(III) and nano-Al13 species on malate dehydrogenase activity

The effects of different aluminum species on malate dehydrogenase (MDH) activity were investigated by monitoring amperometric i-t curves for the oxidation of NADH at low overpotential using a functionalized multi-wall nanotube (MWNT) modified glass carbon electrode (GCE). The results showed that Al(III) and Al₁₃ can activate the enzymatic activity of MDH, and the activation reaches maximum levels as the Al(III) and Al₁₃ concentration increase. Our study also found that the effects of Al(III) and Al₁₃ on the activity of MDH depended on the pH value and aluminum speciation. Electrochemical and circular dichroism spectra methods were applied to study the effects of nano-sized aluminum compounds on biomolecules.

Electrochemical and spectral study on the effects of Al(III) and nano-Al13 species on glutamate dehydrogenase activity

A functionalized multi-wall nanotube (MWNT) modified glass carbon electrode (GCE) was used to study the effects of aluminum species on glutamate dehydrogenase (GLDH) activity by monitoring amperometric i-t curve for the oxidation of the enzymatically generated NADH. The conformational changes of the coenzyme nicotinamide adenine dinucleotide (NAD(+)) induced by Al(III) and nanometer-sized tridecameric aluminum polycation (nano-Al(13)) were investigated by the fluorescence technique. The results showed that the electrochemical method may be a potential tool to investigate the activity of enzymes in the biological system. It may also be useful in studying the effects of nano-sized aluminum compounds on biomolecules in order to discuss their safety to the environment and human.

Synthesis of gelatin-stabilized gold nanoparticles and assembly of carboxylic single-walled carbon nanotubes/Au composites for cytosensing and drug uptake

Gelatin-stabilized gold nanoparticles (AuNPs-gelatin) with hydrophilic and biocompatible were prepared with a simple and "green" route by reducing in situ tetrachloroauric acid in gelatin. The nanoparticles showed the excellent colloidal stability. UV-vis spectra, transmission electron microscopy (TEM), and atomic force microscopy revealed the formation of well-dispersed AuNPs with different sizes. By combination of the biocompatibility of AuNPs and excellent conductivity of carboxylic single-walled carbon nanotubes (c-SWNTs), a novel nanocomposite was designed for the immobilization and cytosensing of HL-60 cells at electrodes. The immobilized cells showed sensitive voltammetric response, good activity, and increased electron-transfer resistance. It can be used as a highly sensitive impedance sensor for HL-60 cells ranging from 1 x 10(4) to 1 x 10(7) cell mL(-1) with a limit of detection of 5 x 10(3) cell mL(-1). Moreover, the nanocomposite could effectively facilitate the interaction of adriamycin (ADR) with HL-60 cells and remarkably enhance the permeation and drug uptake of anticancer agents in the cancer cells, which could readily lead to the induction of the cell death of leukemia cells.

An Effective Amperometric Biosensor Based on Gold Nanoelectrode Arrays

A sensitive amperometric biosensor based on gold nanoelectrode array (NEA) was investigated. The gold nanoelectrode array was fabricated by template-assisted electrodeposition on general electrodes, which shows an ordered well-defined 3D structure of nanowires. The sensitivity of the gold NEA to hydrogen peroxide is 37 times higher than that of the conventional electrode. The linear range of the platinum NEA toward H(2)O(2) is from 1 x 10(-6) to 1 x 10(-2) M, covering four orders of magnitudes with detection limit of 1 x 10(-7) M and a single noise ratio (S/N) of four. The enzyme electrode exhibits an excellent response performance to glucose with linear range from 1 x 10(-5) to 1 x 10(-2) M and a fast response time within 8 s. The Michaelis-Menten constant km and the maximum current density i(max) of the enzyme electrode were 4.97 mM and 84.60 muA cm(-2), respectively. This special nanoelectrode may find potential application in other biosensors based on amperometric signals.

Aqueous suspension of carbon nanotubes enhances the specificity of long PCR

DNA manipulation technology is facing more challenges in the postgenomics era. More and more nanomaterials have been investigated for their potential implications in developing better gene technology. In this study, we reported the beneficial effect of single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) in enhancing the specificity and total efficiency of long (14 kb) PCR. Hydroxylic and carboxylic carbon nanotubes (CNTs) had similar enhancing effects. Nanotubes could become another component for improvements in the amplification of long DNA.

Direct electrochemistry of lactate dehydrogenase immobilized on silica sol–gel modified gold electrode and its application

The direct electrochemistry of lactate dehydrogenase (LDH) immobilized in silica sol-gel film on gold electrode was investigated, and an obvious cathodic peak at about -200 mV (versus SCE) was found for the first time. The LDH-modified electrode showed a surface controlled irreversible electrode process involving a one electron transfer reaction with the charge-transfer coefficient (alpha) of 0.79 and the apparent heterogeneous electron transfer rate constant (K(s)) of 3.2 s(-1). The activated voltammetric response and decreased charge-transfer resistance of Ru(NH(3))(6)(2+/3+) on the LDH-modified electrode provided further evidence. The surface morphologies of silica sol-gel and the LDH embedded in silica sol-gel film were characterized by SEM. A potential application of the LDH-modified electrode as a biosensor for determination of lactic acid was also investigated. The calibration range of lactic acid was from 2.0 x 10(-6) to 3.0 x 10(-5) mol L(-1) and the detection limit was 8.0 x 10(-7) mol L(-1) at a signal-to-noise ratio of 3. Finally, the effect of environmental pollutant resorcinol on the direct electrochemical behavior of LDH was studied. The experimental results of voltammetry indicated that the conformation of LDH molecule was altered by the interaction between LDH and resorcinol. The modified electrode can be applied as a biomarker to study the pollution effect in the environment.

V-type nerve agent detection using a carbon nanotube-based amperometric enzyme electrode

An enzyme electrode for the detection of V-type nerve agents, VX (O-ethyl-S-2-diisopropylaminoethyl methylphosphonothioate) and R-VX (O-isobutyl-S-2-diethylaminoethyl methylphosphonothioate), is proposed. The principle of the new biosensor is based on the enzyme-catalyzed hydrolysis of the nerve agents and amperometric detection of the thiol-containing hydrolysis products at carbon nanotube-modified screen-printed electrodes. Demeton-S was used as a nerve agent mimic. 2-(Diethylamino)ethanethiol (DEAET) and 2-(dimethylamino)ethanethiol (DMAET), the thiol-containing hydrolysis product and hydrolysis product mimic of R-VX and VX, respectively, were monitored by exploiting the electrocatalytic activity of carbon nanotubes (CNT). As low as 2 microM DMAET and 0.8 microM DEAET were detected selectively at a low applied potential of 0.5 V vs Ag/AgCl at a CNT-modified mediator-free amperometric electrode. Further, the large surface area and the hydrophobicity of CNT was used to immobilize organophosphorus hydrolase mutant with improved catalytic activity for the hydrolysis of the P-S bond of phosphothiolester neurotoxins including VX and R-VX nerve gases to develop a novel, mediator-free, membrane-free biosensor for V-type nerve agents. The applicability of the biosensor was demonstrated for direct, rapid, and selective detection of V-type nerve agents' mimic demeton-S. The selectivity of the sensor against interferences and application to spiked lake water samples was demonstrated.

The adsorption–desorption process of bovine serum albumin on carbon nanotubes

The aim of this work is to study the adsorption-desorption process of bovine serum albumin (BSA) on carbon nanotubes (CNT) by reflectometry. The effect of the surface properties was analyzed by comparing the behavior of BSA on silica. The experiments were performed by reflectometry at different BSA concentrations, at pH 3.0, 4.8, and 7.0 and at two ionic strengths. Protein desorption was induced by either dilution with buffer or the addition of SDS. The initial adsorption rate is controlled by the attachment of BSA molecules to the surface, and strongly diminishes at pH 7. Adsorption isotherms reflect the high affinity of BSA for both sorbent surfaces and reach well-defined plateau values that depend on the pH, being the highest at pH 4.8 on CNT. Experiments performed at different ionic strengths (NaCl added) showed a less pronounced effect. Dilution does not induce desorption on either surface however, the addition of SDS removes protein only from the silica surface.

Covalent immobilization of redox enzyme on electrospun nonwoven poly(acrylonitrile-co-acrylic acid) nanofiber mesh filled with carbon nanotubes: A comprehensive study

In this work, novel conductive composite nanofiber mesh possessing reactive groups was electrospun from solutions containing poly(acrylonitrile-co-acrylic acid) (PANCAA) and multi-walled carbon nanotubes (MWCNTs) for redoxase immobilization, assuming that the incorporated MWCNTs could behave as electrons transferor during enzyme catalysis. The covalent immobilization of catalase from bovine liver on the neat PANCAA nanofiber mesh or the composite one was processed in the presence of EDC/NHS. Results indicated that both the amount and activity retention of bound catalase on the composite nanofiber mesh were higher than those on the neat PANCAA nanofiber mesh, and the activity increased up to 42%. Kinetic parameters, K(m) and V(max), for the catalases immobilized on the composite nanofiber mesh were lower and higher than those on the neat one, respectively. This enhanced activity might be ascribed to either promoted electron transfer through charge-transfer complexes and the pi system of carbon nanotubes or rendered biocompatibility by modified MWCNTs. Furthermore, the immobilized catalases revealed much more stability after MWCNTs were incorporated into the polymer nanofiber mesh. However, there was no significant difference in optimum pH value and temperature, thermal stability and operational stability between these two immobilized preparations, while the two ones appeared more advantageous than the free in these properties. The effect of MWCNTs incorporation on another redox enzyme, peroxidase, was also studied and it was found that the activity increased by 68% in comparison of composite one with neat preparation.

The advantage of using carbon nanotubes compared with edge plane pyrolytic graphite as an electrode material for oxidase-based biosensors

Carbon nanotubes (CNTs) are promising materials for use in amperometric biosensors. The defect sites at their ends, and on their sidewalls, are considered to be edge plane-like defects and show high electrocatalytic activity toward several biological molecules. However, electrocatalytic activity toward H(2)O(2) has not been compared among bamboo-structured CNTs (BCNTs), which have many defect sites; hollow-structured CNTs (HCNTs), which have few defect sites; edge plane pyrolytic graphite (EPG); and traditional glassy carbon (GC). The advantages of using CNTs in electrodes for biosensors are still equivocal. To confirm the utility of CNTs, we analyzed the electrochemical performance of these four carbon electrodes. The slope of the calibration curve for H(2)O(2) at potentials of both +0.6 V and -0.1 V obtained with a BCNT paste electrode (BCNTPE) was more than 10 times greater than the slopes obtained with an HCNT paste electrode and a GC electrode, reflecting the BCNT's larger number of defect sites. Although the slope with the EPG electrode (EPGE) was about 40 times greater than that with BCNTPE at +0.6 V, the slopes with these two carbon electrodes were nearly equivalent at -0.1 V. EPGE demonstrated excessive electrochemical activity, detecting currents on the basis of consumption of oxygen and oxidation of ascorbic acid, even at -0.1 V. In contrast, BCNTPE could dominantly detect a cathodic current for H(2)O(2) at -0.1 V, even when interfering molecules were added. BCNTPE possesses appropriate electrochemical activity and is an effective electrode materials for developing interference-free oxidase-based biosensors operated by the application of an appropriate potential.